FIGS. 6(a)-6(e) are sectional views illustrating process steps for connecting an electrode and a wiring layer in a conventional method of producing a compound semiconductor device.
Initially, a front surface of a GaAs substrate 1 is covered with a first insulating film 2, and a resist pattern 3 is formed on the insulating film 2 using a conventional photolithographic technique. Using the resist pattern 3 as a mask, the first insulating film 2 is dry etched to expose a region of the GaAs substrate 1 where an electrode is to be formed (FIG. 6(a)).
In the step of FIG. 6(b), Au-Ge alloy, Ni, and Au are successively deposited over the entire surface of the GaAs substrate 1 to form an electrode metal film 4 comprising Au-Ge/Ni/Au. Then, the resist pattern 3 and overlying portions of the metal film 4 are removed using a lift-off technique, resulting in an electrode 4a as shown in FIG. 6(c).
In the step of FIG. 6(d), the first insulating film 2 and the electrode 4a are covered with a second insulating film 7, and a contact hole 7a is formed penetrating through the second insulating film 7 using conventional photolithography and etching technique. Then, an Au film 6 for wiring is deposited over the substrate.
Thereafter, as illustrated in FIG. 6(e), the Au film 6 is patterned in a desired shape using an etching process, resulting in a wiring layer 6a.
In the above-described process steps, since the metal film 4 comprises Au-Ge/Ni/Au, the electrode 4a produces an ohmic contact with the GaAs substrate 1. Since the metal film 6 for wiring comprises Au, the wiring layer 6a is connected to the electrode 4a with high thermal and mechanical stability. In addition, the contact resistance between the wiring layer 6a and the electrode 4a is reduced.
With increasing requirements for small-sized and highly-integrated semiconductor devices in recent years, finer wiring patterns have been needed. In the conventional compound semiconductor device, however, since Au is employed for the wiring layer 6a, which is patterned by sputter etching or ion beam etching but cannot be patterned by reactive ion etching (RIE) that provides a fine patterning, a fine wiring pattern is not attained. Since Al can be patterned by RIE, it should be thought that an Al wiring layer is employed in the compound semiconductor device. In this case, however, if the device is subjected to a heat treatment at about 300.degree. C. after the formation of the Al wiring layer, an undesirable solid phase reaction of Al and Au occurs at the junction between the Al wiring layer and the ohmic electrode formed on the GaAs substrate, i.e., the Au-Ge/Ni/Au electrode 4a, whereby an intermetallic compound, such as AuAl.sub.2, is formed at the junction. The intermetallic compound increases the contact resistance and reduces the mechanical strength, adversely affecting the performance and reliability of the device.
On the other hand, in a semiconductor device employing an Si substrate, if Au is in contact with the Si substrate, trapping centers for trapping carriers are formed within the Si substrate, adversely affecting the semiconductor device characteristics. In order to make a favorable ohmic contact with the Si substrate, an Al or Al base alloy is usually employed for the electrode and an Au wire is bonded on the electrode. The Au wire is employed in the bonding process because a nail head bonding process which is not restricted in the bonding direction can be adopted using the Au wire in a package including a plurality of leads, these leads can extend in more than one direction, whereby a favorable connection between the Au wire and each lead is achieved.
As described above, if heat is applied to the junction between Al and Au, Al and Au easily react with each other. Therefore, also in the above-described semiconductor device, when the Au wire is bonded on the Al electrode, an intermetallic compound, such as AuAl.sub.2, is formed at the junction, increasing contact resistance and reducing mechanical strength. In order to suppress the generation of the intermetallic compound, the initial stage in the bonding process is conventionally carried out in a time as short as possible at a low temperature to reduce the solid phase diffusion between Au and Al. However, no matter how short the bonding time at the low temperature bonding is, Au and Al easily react with each other, so that it is impossible to completely prevent the generation of the intermetallic compound.